Most of copy machines useful in office work employ an electrophotographic copy system. A typical mechanism of such electrophotographic system may be represented as follows: A photoconductive layer is photosensitized by being charged with a surface potential; through an exposure to light a latent electrostatic image of an original image is formed on the photoconductive layer; subsequently, a toner is charged with a potential having a charge opposite to that of the potential of the latent image; the charged toner is transferred onto the latent image and then transferred again onto a plain paper by electrostatic force, on which the toner image is fixed to provide a desired printed image.
In electrophotography, a hot roll fixing method has been generally used to fix a toner image on a plain paper. The hot roll fixing method comprises transferring a toner image from the surface of a photoconductive drum in an electrophotographic copying machine to a plain paper and fixing permanently the toner image on the paper using a hot roller at a temperature ranging from 120.degree. to 180.degree. C.
Toners which can be suitably used in the hot roll fixing method are required to soften at relatively low temperatures and adhere easily to the paper(to meet the fixability requirement) and not to adhere to the hot rollers even when the temperature of the hot rollers is relatively high(to have the non-offset property). The above-mentioned properties of toners are mainly dictated by the binder resins used for the preparation of the toners.
In general, if the melt viscosity of a binder resin used in a toner decreases, the fixability of the toner upon hot fixing becomes better while the non-offset property of the toner becomes deteriorated and vice versa. Therefore, many attempts have been made to develop a binder resin capable of satisfactorily meeting the above requirements for both fixability and non-offset property.
U.S. Pat. No. 4,486,524 discloses a method which comprises: producing each of a low molecular weight polymer which imparts the fixability to a toner and a high molecular weight polymer which gives the non-offset property to the toner separately; and mixing the polymers so produced. This method improves the fixability of the toner; but has the defect that the toner has poor non-offset property; and, further, a long period of time is required to produce the high molecular weight polymer, and the cost of production is high. U.S. Pat. No. 4,652,511 discloses a process for producing a resin composition which comprises suspension-polymerizing at least one vinyl monomer in aqueous medium in the presence of a dispersing agent and a high molecular weight vinyl polymer obtained by emulsion polymerization. This process requires short periods of time to produce a toner resin but the non-offset property still remains inferior.
Japanese Laid Open Patent Publication No. 134,248/1985 proposes a two-step polymerization which produces a polymer of partially cross-linked structure, that is, having both a cross-linked structure and a linear structure of low molecular weight. However, in this method, non-offset property and fixability are also liable to betray each other depending upon the cross-link density of the polymer. That is, when the cross-link density of the polymer is low, the proper gel content can not be maintained and the non-offset property of toner is poor although the fixability is adequate. On the other hand, when the cross-link density of the polymer is high, the non-offset property becomes acceptable while the toner's fixability becomes inferior.
Further, European Patent Publication No. 412,712 discloses a method which comprises: blending a low molecular weight resin containing an epoxy group and a pseudo cross-linked resin prepared by introducing an unsaturated carboxylic acid into a low molecular weight resin and attaching a multivalent metal thereto; and cross-linking the carboxylic acid with an epoxy group during a compounding process for preparing a toner, to improve-the non-offset property. However, the pseudo cross-linked resin in the form of a complex with the multivalent metal has poor heat stability during the compounding process; and, further, it is difficult to maintain a high viscosity of the pseudo cross-linked resin, which determines the ability to improve the non-offset property, and, furthermore, the efficiency of improving the non-offset property tends to be inconsistent because of the non-uniform complex formation reaction during the compounding process.